1. Field of the Invention
The present invention relates to a Roots type pump, particularly, one adapted for use as a mechanically driven supercharger in an internal combustion engine.
2. Description of the Related Art
It is well known that a Roots type pump has a pair of mating lobed rotors which synchronously rotate with each other, with a slight clearance therebetween. When the Roots type pump is used as a mechanically driven supercharger in an internal combustion engine, the rotational speed of the rotor varies over a wide range, depending upon the rotational speed of the engine. This tends to cause the rotors to make contact with each other, resulting in noise. In addition, foreign particles may clog the rotors between the slight clearance, resulting in damage to the rotors. To solve these problems, it is known to provide a coating layer of plastic material on the outer surfaces of the rotors (for example, Japanese Unexamined Utility Model Publication No. 59-81793).
A further problem occurs when the plastic coating is applied on the outer surfaces of the rotors, in that the plastic coating becomes detached from the outer surfaces of the rotors. Particularly, the rotors of the Roots type pump have a special cocoon-like shape in cross section, i.e., a lobed long diameter portion along the major axis and a narrow short diameter portion along the minor axis perpendicular to the major axis, which causes the tension on the plastic coating in the direction of the major axis of the rotor through thermal deformation of the plastic coating and the rotor, and thus the coating layer tends to peel away from the rotor at the narrow portions of the rotors.
A further problem occurs when the rotor is fixed to the rotor shaft by a pin (for example, as in Japanese Unexamined Utility Model Publication No. 59-165987), which is generally inserted in a through hole provided through the rotor and transversely extends between the narrow short diameter portions. FIG. 5 of the attached drawings shows an exaggerated view of a portion of a rotor of a Roots pump in which a pin 14 is inserted in a hole in a rotor 2 in the direction indicated by the arrow. When the pin 14 is forcibly inserted in the hole, the inner wall of the hole is deformed, although the extent of the deformation is minute. Nevertheless, further insertion of the pin produces a larger deformation, and when the leading end of the pin has been fully inserted in the hole, the wall of the hole near the opening end thereof is deformed outwardly of the hole and the outer surface of the rotor is pushed up even though the pin does not project from the outer surface of the rotor. This outward deformation, i.e., the bulge in the outer surface of the rotor, causes a corresponding bulge in the coating layer of plastic material 10. The peeling of the coating layer due to the special shape of the rotor, as described above, tends to start at this bulged region, and thus it appears that this bulge in the coating layer may lead to the actual, undesirable peeling of the coating layer.